1. Field of the Invention
The present invention relates to coating compositions, which preferably dry at room temperature, contain alkoxysilane groups and are based on polyisocyanates prepared from 4,4'-diisocyanatodicyclohexyl-methane, and to their use as a protective coating.
2. Description of the Prior Art
Damage to coatings due to the penetration of dyes, caused by graffiti, for example, constitutes an increasing problem. Conventional organic coatings swell due to the solvent contained in the lacquer spray and, thus, can readily be penetrated by dyes. It is then no longer possible to clean the surface without destroying the coating.
Therefore, there is a need for coatings which form an effective barrier to solvents and dyes and which thus protect the underlying substrate from contamination.
Coatings that can be removed with the dye or with the contaminant (e.g. FR-A 2,747,325, WO 97/24407), do provide satisfactory protection, but involve considerable cost, since they have to be freshly applied again and again.
Permanent coatings can be treated with silicone polymers or fluoropolymers in order to decrease their surface energy (e.g. EP-A 695 772, FR-A 2,681,072), so that wetting of the surface is made more difficult. A disadvantage of this treatment is that the fluoro- or silicone polymers, which are not chemically bonded, are washed out over time due to weathering effects and lose their effectiveness. In contrast, chemically bound polymers result in incompatibility effects, haze effects and poor adhesion to the substrate, so that decorative clear coatings, which exhibit a high level of transparency, cannot be treated in this manner.
In contrast, ultra-hard sol-gel coatings (e.g. EP-A 576 166) provide good protection from graffiti. However, these systems have to be stoved at elevated temperatures, so that they are unsuitable for use for coating large vehicles, for example. Also, these coating compositions exhibit poor adhesion, particularly to other coatings, and often exhibit unsatisfactory resistance to the impact of stones.
Reaction products of aminosilanes with polyisocyanates (silane-functional ureas) are known and can be used, for example, for the production of flexible coatings (U.S. Pat. Nos. 5,700,868, 4,625,012, 4,474,933, 3,979,344, DE-A 4,234,325, DE-A 2,155,259). Since these products for the most part are based on high molecular weight polymers and their density of crosslinking is low, these coatings are less suitable as protective coatings against color-imparting liquids. In principle, however, silicone-functional ureas can be used for the production of sol-gel coatings with a high density of crosslinking. Their disadvantages are their tendency to crystallize, which is often pronounced, and/or their rate of hardening at room temperature, which is often so low that an efficient protective coat has not formed even several days after application.
An object of the present invention is to provide a protective coating which preferably does not have to be stoved and 24 hours after application at room temperature has hardened to such an extent that the penetration of dyes (e.g. graffiti) is effectively prevented. An additional object is to provide a coating that has good visual quality and exhibits good scratch resistance such that it is suitable for use an automotive coating.
These objects may be achieved in accordance with the siloxane-curing coatings of the present invention, which are based on highly functional 4,4'-diisocyanato-dicyclohexylmethane polyisocyanates and form an efficient barrier layer against dyes from graffiti within 24 hours after application. These coatings can be applied to any substrate, exhibit good scratch resistance, and provide effective protection from contaminants.